1. Field of the Invention
The present invention relates to an electronic ballast for starting and operating high intensity discharge (HID) lamps using a new, low energy loss circuit arrangement connected across a common low voltage AC power source which provides improved efficiency when contrasted with conventional HID lamp ballasts.
2. Discussion of the Background
Prior art HID ballast circuit such as disclosed in U.S. Pat. No. 4,337,417 utilize transformers connected in series to an input AC voltage source at one end and to an output terminal of a HID lamp at the other end. Capacitors and charging resistors as well as blocking diodes are utilized in order to effect high voltage starting pulses for lamp ignition. Ignition occurs when a capacitor is initially charged to the peak voltage of the AC source during the negative half cycle of the source and then when the source voltage goes negative the voltage of the first capacitor is added to a second capacitor in order to provide a voltage of twice the AC input source voltage. A transformer utilizes discharge energy and applies a voltage pulse of sufficient magnitude across a lamp. This type of prior art suffers from a lack of efficiency because of energy loss in the circuit. Most energy loss occurs in the transformers which generate high heat losses. Thus there is critical need to more efficiently start and operate HID lamps without the high energy losses which are characteristic of the conventional ballast circuits using a high loss element.
Other prior art devices have attempted to address this high loss problem. One approach is the "lead ballast" circuit structure such as shown in U.S. Pat. No. 3,710,184 wherein a low energy circuit is used to cause an open circuit voltage (OCV) for lamp ignition to be increased. This type of system also has energy losses which cause it to provide less than an optimal solution.
Another approach is taken in the U.S. Pat. No. 3,700,962 of Munson which utilizes a low voltage high energy source but which does not provide any measure of taking into account the dynamic impedance of the discharge necessary with HID lamps. That is, many discharge lamps have dynamic specific needs which cannot be addressed by a single application of a voltage or a single application of one single specific amount of energy.
Thus there remains a need to more efficiently start and operate HID lamps without the high energy losses which are characteristic of conventional ballast circuits using a high loss element. There is also a simultaneous need to operate HID lamps using systems which are capable of taking into account the dynamic impedance requirements for HID lamps without a substantial loss of efficiency.